1. Field of the Invention
The present invention relates generally to a thread form for tubular connections, and, more specifically, to threaded connections of the type used for securing flow conduits to form a desired continuous flow path.
2. Description of the Prior Art
A variety of threaded connections are known in the prior art for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid. For example, such threaded connections are used in pipe strings employed for the production of hydrocarbons and other forms of energy from subsurface earth formations. Examples of such pipe strings include drill pipe, well casing and production tubing, known commonly as xe2x80x9coil field tubular goods.xe2x80x9d Other applications for the threaded connections of the invention include horizontal/trenchless drilling operations. These are non-oilfield applications associated with the construction industry. All of these type goods employ threaded connections of the type under consideration for connecting adjacent conduit sections or pipe joints.
There have been numerous advances in thread technology in recent years. U.S. Pat. No. Re. 30,647 issued to Blose in 1981 disclosed a tubular connection having a unique thread form which provided an unusually strong connection while controlling the stress and strain in the connected pin and box members of the connection. The thread form featured mating helical threads which were tapered in thread width in opposite directions to provide wedge-like engagement of the opposing flanks to limit rotational make-up of the connection.
The wedge thread, if properly designed, provides high torsional resistance without inducing axial or radial stresses into the tubular connection upon make-up of the joint. Tubular connections with high torsional resistance resist additional make-up in the joint when in service, making it easier to break out the joints if this becomes necessary. By reducing axial or radial stresses in the threaded connection, a sounder connection is provided which is able to withstand a greater level of operating stress and strain. Whereas, traditional threaded connections employed mating threads structures which placed in the pin in hoop compression and the box in hoop tension, the Blose connection controlled the stresses induced in the mating members to pull the two members together, rather than apart.
U.S. Pat. No. 4,600,224, issued Jul. 15, 1986 to Blose was a refinement and further improvement to the basic wedge thread concept. In the invention disclosed in the ""224 patent, a connection was shown having a xe2x80x9cchevronxe2x80x9d load flank. Radial make-up of the threaded connection was controlled by the special thread structuring where the radial movement of a thread into a mating thread groove was restricted by a chevron type interfit between two load bearing thread surfaces of the threaded connection instead of relying upon thread width alone.
Due to the helical wedging action of the threads and the balance of radial components from the forces on the stab and load bearing surfaces of the threads, no radial component of stress was induced in either the pin or box member of the connection. It was also possible to control axial make-up of the threaded connection without abutting a shoulder on the joint through the wedging action of the threads themselves. By controlling the radial interfit between mating threads, lubricant entrapment could be controlled.
U.S. Pat. No. Re. 34,467 issued Dec. 7, 1992 to Reeves purported to be an improvement to the basic Blose wedge thread design. As explained by the patentee, when Blose""s connection is rotatably made up to engage both the front and back thread load flanks, incompressible thread lubricant or other liquid may be trapped between the engaged load flanks. This trapped thread lubricant can resist the make-up torque and give a false torque indication that results in lower than desired stress and strain being induced in the Blose connection and reducing the design strength and load carrying capacity. The invention described in U.S. Pat. No. Re. 34,467 purports to preclude the possibility of false indication of torque by excluding thread lubricant from between the thread load flanks that are brought into engagement at make-up.
In the conventional prior art connection, only a single load flank is typically engaged during make-up and clearance is provided adjacent the back thread flank. By providing thread clearance on the back flank of conventional thread structures, a helical escape flow passage or reservoir is provided for receiving any excess of displaced liquid thread lubricant during make-up. Since Blose""s threads were structured helically as a xe2x80x9cwedgexe2x80x9d, it was necessary to provide a radial clearance between the thread crests and roots upon make-up to provide the desired escape path or any entrapped lubricant. U.S. Pat. No. Re. 34,467 purports to eliminate the problem of entrapped lubricant by optimizing the geometry of the Blose wedge thread structure.
As stated by the patentee, the root wall on the external thread member contacts the crest walls on the internal thread member to exclude the trapping of liquid therebetween during rotational make-up of the connection. In the earlier Blose connection, the roots and crests of the threads did not engage. Thread lubricant could be trapped in the long helical space between the roots and crests of the thread. In U.S. Pat. No. Re. 34,467, the threads were modified so that the roots and crests would engage before the load flanks engaged, thereby causing the thread lubricant between the roots and crests to be squeezed out from between the surfaces before the connection is fully made up.
In U.S. Pat. No. Re. 30,647 and U.S. Pat. No. Re. 34,467, the preferred threads were xe2x80x9cdovetailed-shapedxe2x80x9d in cross section, being wider at the crests than at the roots. U.S. Pat. No. 4,600,224 was a departure from the Blose design in that a semi-dovetail or partial dovetail thread was disclosed. However, the thread crest width continued to be greater than the thread root width as in the traditional definition of the term xe2x80x9cdovetail.xe2x80x9d
The present invention has as its object to provide a further modification of the basic wedge thread concept which provides improved performance over the prior art designs.
The improved thread form of the invention controls radial make-up by special thread structuring where the radial movement of a thread into a mating thread groove is controlled by a complex profile interfit between the two mating thread surfaces of the threaded connection. The complex profile can be present on the stab flank, on the load flank, or on a combination of the two flanks. A controlled clearance is provided between the mating crests of the interengaged threads to prevent hydraulic pressure buildup caused by entrapped lubricant between the thread crests and roots. The stab flanks complex profile can be a chevron-shaped double sloped flank which is adapted to engage a mating flank of the complimentary box member. Preferably, the complex profile provided on the stab and/or load flanks of the thread form is a multi-facited flank having at least three facits and four radii per stab flank. The pin thread crests have a crest width and the pin roots have a root width. The width of the crest is less than the width of the roots, which is exactly opposite that of the general dovetail design.
The thread form of the invention is intended for use in a tubular connection of the type having a box with internal threads with stab flanks and load flanks and flat roots and crests and a pin having external threads with stab flanks and load flanks and flat crests and roots for mating with the internal threads of the box to make up a pipe connection. The threads increase in width in one direction on the box and in the other direction on the pin so that the roots, crests, and flanks of the threads move together during make-up.
The pin thread has a complex profile stab flank made up of at least an inner wall portion and an intersecting outer wall portion which together form a double sloped flank for engaging a mating stab flank of the complimentary box member to provide an interfit between the stab flanks of the pin and box members. The interfit serves to limit radial movement of the pin threads into the mating box member complimentary thread structure to control radial make-up of the pipe connection. Preferably, the complex profile stab flank is a multi-facited flank having at least three facits and four radii per stab flank. In a particularly preferred embodiment, the pin thread also has a complex profile load flank for engaging a mating load flank of the complimentary box member. The interfit of the pin and box stab flanks and load flanks provides a clearance between the roots and crest of the pin and box threads to allow venting of any entrapped thread lubricant.
A threaded tool joint adapter connection having encapsulated stresses is also shown. The adapter connection features an intermediate member with oppositely facing pin ends. The oppositely facing pin ends engage mating box members which can be used to attach a standard, rotary tool joint connection to upset pipe. In one embodiment, the intermediate member is formed of a corrosion resistant alloy. The box members are formed of NACE approved materials. The pin and box threads can have the previously described complex stab and load flanks to provide the desired interfit between the flanks of the pin and box members or can have other wedge thread profiles. A metal-to-metal seal is formed between mating surfaces of the box and pin and opposite extents of the threaded surfaces thereof at either end of the connection. The metal-to-metal seals serve to encapsulate the threaded surfaces of the connection and thereby isolate the encapsulated surfaces from corrosive attack.
Additional objects, features and advantages will be apparent in the written description which follows.